This is a Class D
or switching amplifier
circuit. It amplifies the 30 Hz signal to the upper left. First, it compares
the input to a high-frequency triangle wave, using a
op-amp
as a comparator. It uses the output of the comparators to drive two
MOSFETs which bring their
output to +15 V if the output is higher than the triangle wave, or -15
V if it's lower. Since the triangle wave sweeps up and down at high
frequency, we get a series of spikes, which are higher on average when
the input is higher. An LC filter
smoothes out the spikes to generate the amplified output.